Computed tomography (CT) systems typically include an x-ray source collimated to form a fan beam directed through an object to be imaged, i.e. a patient, and received by an x-ray detector array. The x-ray source, the fan beam and the detector array are oriented to be situated within the x-y plane of a Cartesian coordinate system, termed the “imaging plane”. The x-ray source and the detector array may be rotated together on a gantry within the imaging plane, around the imaged object, and hence around the z-axis of the Cartesian coordinate system.
In computed tomography systems, a device called collimator is generally used to minimize the x-ray radiation dose a patient receives. One of the ways to achieve this goal is to insert a bowtie-shaped piece of polymer, called “wedge”, in the path of the x-ray beam. The wedge, functioning as an x-ray attenuation filter, is generally a synthetic polymer such as Teflon having an x-ray absorption spectral characteristic near that of water and hence the human body. The attenuation filter is intended to compensate for the variation in thickness of the imaged body. The x-rays that pass through the center of the imaged body, normally the thickest part, are least attenuated by this filter, whereas the x-rays that pass through the edges of the imaged body, normally the thinnest part, are more attenuated by this filter. The result of this selective attenuation is that the x-rays impinging on the detectors have a similar energy and are centered midway around the sensitivity of the detector. The attenuation filter may therefore allow use of more sensitive x-ray detectors reducing the range of x-ray energies. Over time, and under continued x-ray exposure, the mechanical characteristics of the polymer change. The x-rays may degrade the polymer, causing it to become brittle. In combination with mechanical stresses, this can cause cracks which introduce discontinuity of the x-ray beams and may thus cause severe image artefacts.
Prior-art document U.S. Pat. No. 4,975,933 discloses a bowtie x-ray filter assembly for dual energy tomography. According to this disclosure, an attenuation filter is mounted to reduce expansion-induced stress. Particularly, the filter element for the attenuation filter is attached to a movable support plate for positioning the filter element within the x-ray beam. The center of the filter element is affixed to the support plate but the ends are attached so as to slide with changes in temperature and thus with changes in the dimensions of the filter element and the support plate.
FIG. 1A is a diagram of an attenuation filter 10 used in the prior art, for example, the attenuation filter as described in U.S. Pat. No. 4,975,933. FIGS. 1B and 1C are a schematic front view and a cross-sectional view, respectively, corresponding to the attenuation filter 10. The attenuation filter 10, e.g. the polymer wedge, is made of a rectangular filter block 12 of Teflon.
Situated in the exposed face (i.e. the face receiving the x-ray beam along axis 11) of the filter block 12 is a saddle notch 14 extending in the center within the filter block 12 but through less than the full width of the filter block 12 so as to leave a supporting wall 16 intact along the path of the projected fan beam of x-ray 18 radiating vertically along axis 11 so that the thickness of the attenuation filter 12 corresponds inversely to the thickness of a typical object being imaged (not shown). The attenuation filter 10 is thinner in its center so as to attenuate x-rays 18 passing through the thickest portion of the imaged object, and thickest at either edge so as to attenuate most x-rays 18 passing without any attenuation on either side of the imaged object. The purpose of the attenuation filter 10 is to equalize, approximately, the intensity of the x-ray 18 received by a CT detector and hence to allow improved detector sensitivity. Generally, the unexposed face of the attenuation filter 10 is attached to a rectangular support plate (not shown), which positions the attenuation filter 10 within the x-ray beam 18.
U.S. Pat. No. 4,975,933 provides a solution to combat the crack introduced by expansion with temperature. The attenuation filter in the prior art is shaped in such a way that a downwardly extending saddle notch is situated in the exposed face of the filter and is centered within the filter block but through less than the full width of the filter block so as to leave a supporting wall intact. Space, which is at a premium on the rotating part of the computed tomography system, is the price of keeping the supporting wall intact. Intactness of the supporting wall will stop any workpiece from moving through the saddle notch and make movement of the attenuation filter relative to a stationary workpiece in a collimator difficult. Furthermore, the solution provided in the prior art cannot combat the crack introduced by centrifugal forces, which is known as a large source of mechanical stress causing cracks in the attenuation filter.
Therefore, there is a need to provide a filter assembly which is improved with respect to the prior-art filter assembly as described above.